Method of bonding an array of optical fibers

ABSTRACT

A coupler utilizing a repeater. Dedicated transmit and receive lines interconnect transmitters and receivers with the data bus and, receive lines from other hybrid lines are optically isolated by approximately 23 db from the receiver in the repeater by first and second mixers, isolation of the receiver from signals originating from other couplers preventing race conditions. Power equalization is inherent since signals from other couplers are received at nearly the same signal level as signals from local transmitters, repeater transmitter levels being adjustable for compensation for any small difference occurring due to coupler power splitting and line losses.

This is a division, of application Ser. No. 26,372, filed Apr. 2, 1979,now U.S. Pat. No. 4,261,641.

This invention relates to optical transmission medium data bus couplersand more particularly to active multiport optical conductor data buscouplers.

Prior efforts to achieve an active fiber optic data bus have faced thecommon problem that by the very nature of the data bus a signaltransmitted from any terminal gets distributed throughout the data busdistribution network. If a repeater is inserted at some point in thenetwork, means must be provided for insuring that a repeaters receiverdoes not see its own transmission since positive feedback could occurwith the repeater either oscillating or saturating. In either case theeffect is to jam the bus. Since Fresnel reflections occur at allconnector interfaces, it becomes easy to develop sneak paths from whichpositive feedback can result. Accordingly, it is an object in thesuccessful use of repeaters in a data bus distribution network toprovide a substantial amount of loss between a repeaters transmitter andits receiver.

The utilization of multiplex data buses in aircraft, ships and othervehicles provides for reduction in wiring and the achievement ofadditional systems flexibility. Fiber optic buses offer severaladvantages over conventional wire data buses. Large signal bandwidthsand immunity to electromagnetic interference are well known propertiesof fiber optic cables. Further attributes include: freedom fromdisabling short circuits and intermittent connections; completeelectrical isolation of interconnected systems; safety in explosiveenvironments; light weight, and potential low cost.

Essentially all present serial data buses employ T couplers dispersedalong a wire transmission line. This convenient prior art configurationimposes severe limitations on the fiber optic data bus. Series lossthrough many couplers limits the fiber optic data bus to few (10-15)terminals utilizing current state of the art technology. A priorconfiguration most commonly utilized in fiber optic data buses is thestar configuration. In the star configuration, all fiber optic cablesare brought together at a single point at which a signal from any one isdistributed to all ports equally. Loss performance of the star coupleris close to optimum, but the resulting cable configuration is notconvenient for aircraft or ship-board applications where equipmentcenters are widely separated or cable runs are severely restricted. Athird configuration termed hybrid configuration provides for moreconvenient interconnection by the strategic location of a multiportcoupler in each main equipment center and by interconnection of couplerswith a single harness. However, the loss between terminals in theaforementioned hybrid configuration is at best 5 to 7 db greater thanthe loss in a star configuration having the same number of terminals.

It is believed that the above general understanding of the priorapproaches will lead to a better understanding and appreciation of thehereinafter described embodiments of the present invention. Morespecific examples of prior art include: U.S. Pat. Nos. 3,936,141, and3,883,217 which however do not relate to couplers utilizing activeelements; and, U.S. Pat. No. 4,027,153 which in FIG. 1A has hot treatedthe problem of positive feedback which can result in oscillations or acontinuous "on" condition, which problem is addressed hereinafter inconnection with the description relating to figures of the drawingsrepresentative of embodiments of the present invention.

It is accordingly an object of the present invention to: with theconstraint of having a completely passive bus (no repeaters) removed,provide a data bus distribution network which provides the convenientinterconnection capability of the hybrid configuration while maintainingan effective loss between terminals that is lower than the starconfiguration.

Briefly, in accordance with a preferred embodiment of the presentinvention, an active multiport fiber optic data bus coupler is providedin which selected incoming signals are optically isolated from thereceiver of an associated repeater. More particularly, the presentsystem embodiment comprises a first mixer which accepts inputs from aplurality of local transmitters and divides its output into a firstportion that is supplied to an active receiver with its associatedrepeaters, and a second portion that is supplied to a second mixer whichalso accepts inputs from other couplers and combines the signals foroutputing to a plurality of local receivers.

Further description of these and other novel features of the inventionand its principles of operation and of additional examples thereof willbe presented below in connection with a discussion of the accompanyingdrawings given by way of example and in which:

FIG. 1 is a schematic diagram of an active multiport coupler utilizingan optical repeater in accordance with an embodiment of the presentinvention;

FIG. 2 is a perspective view of an optical mixer suitable for use in theactive multiport coupler of FIG. 1;

FIGS. 3A and 3B when placed side by side are illustrative of theschematic for the optical repeater shown in FIG. 1;

FIG. 4 is a block diagram illustrative of a 48 terminal hybridconfigured data bus utilizing four of the 12 terminal active multiportfiber optic data bus couplers of the type shown in FIG. 1;

FIG. 5 is illustrative of the connector arrangements utilized in theactive multiport fiber optic data bus coupler of FIG. 1 and couplers ofthe data bus of FIG. 2;

FIG. 6 is illustrative of signal paths to and from and between a pair ofthe present active data bus couplers;

FIG. 7 shows fixtures utilized in a method of making fiber and mixerassemblies;

FIG. 8 is illustrative of fiber assembly manufacture;

FIG. 9 is illustrative of mixer assembly manufacture;

FIG. 10 is illustrative of coupler assembly manufacture; and

FIG. 11 is a perspective view of final coupler assembly.

Turning now to active multiport fiber optic data bus coupler 88 of FIG.1 (and FIG. 5 showing a connectorized version of FIG. 1) it will beobserved that selected input signals 111 (three are shown) are opticallyisolated from receiver 91 of optical repeater 93. More particularly,active multiport fiber optic data bus coupler 88 comprises first mixer89 which is responsive to a plurality of input signals 101 from aplurality of local transmitters (not shown) and divides its output intofirst output signals 103 via output fibers 105 (shown in FIG. 1 asparallel lines so as to increase the signal to noise ratio) to receiver91 of optical repeater 93, and second output signals 105 coupled toprovide input signals to second mixer 90. Second mixer 90 is alsoresponsive to input signals 111 from other couplers (not shown)combining these signals and providing output signals 113 via outputfibers from second mixer 90 to local receivers (not shown). Opticaltransmitter 92 (seen in more detail in FIG. 5 to include a plurality ofsignal sources denoted T_(x)) output signals 115 are transmitted toother couplers 88 (not shown in FIG. 1 but shown in the data busarrangement of FIG. 4).

In summary, briefly returning now to FIG. 1 and coupler 88 utilizingoptical repeater 93, it will be seen in the embodiment shown that twelveinput signals 101 and twelve output signals 113 on individual lines arepaired (in the form of two channel cables as shown in FIG. 4) tointerconnect transmitters and receivers with the data bus and also tointerconnect the couplers (as 6 pairs of dedicated transmit and receivelines represented in FIG. 4 by leads with double arrowheads). Coupler 88distributes signals from local transmitters and also incoming signalsfrom other couplers to the local receivers. A portion 105 of the signalfrom local transmitters is tapped off to optical repeater 93, whichboosts the signal level prior to transmission to other couplers. Animportant feature of the present system is that optical transmissionlines carrying incoming signals from other couplers are opticallyisolated (by about 23 db optical) from receiver 91 of optical repeater93 by first and second mixers 89 and 90. In coupler 88 of FIG. 1,signals 111 from other couplers are coupled as inputs to second mixer 90downstream from output signals 103 coupled as inputs to optical repeater93. Isolation of receiver 91 from signals originating from othercouplers is essential in avoiding race conditions. Preliminarycalculations indicated that the desired signal (to be repeated) toundesired signal (not to be repeated) of an optical 21 db would resultfrom this configuration which is more than sufficient for high qualityreception and the prevention of race conditions. Power equalization isinherent in this embodiment since signals from other couplers arereceived at nearly the same signal level as signals from localtransmitters, any small difference occurring due to coupler powersplitting and lire losses being easily compensated for by adjustment ofrepeater transmitter levels.

Optical mixers 89 and 90 are optical waveguides whose function is toaccept optical power at any point on their input and distribute ituniformly over their output and may comprise an exemplary component suchas shown in FIG. 2 (with appropriate legends shown for clarity). Opticalmixers 89 and 90 include a core of optically conducting materialsurrounded over their lengths by a material having a lower index ofrefraction than the core material so that light entering the mixersinput is substantially completely reflected off the sides. The shape ofthe mixer is an important factor in determining the degree of uniformitythat power is distributed over the mixers output. For example, skew raysinjected into cylindrical type mixers (not shown) tend to follow helicalpaths which never pass through the center of the mixer. Mixers havingrectangular cross sections (such as shown in FIG. 2) do not have theskew-ray problem and give better uniformity. In the rectangular crosssection mixer of FIG. 2, distribution of rays plotted at intervals alongthe length of the mixer is illustrated, the number of rays at eachrectangular cross section 303 has been adjusted to give good visualcontrast so that FIG. 2 can illustrate how optical power from any one ofthe input fibers can be divided equally between all of a multiplicity ofoutput fibers.

Optical repeater 93 of FIG. 1 is shown in detailed schematic form inFIGS. 3A and B and generally comprises means for detecting a low leveloptical signal, amplifying the signal, discriminating between signals tobe repeated (or regenerated) an undesired noise or other signals, andgeneration of the desired data bus waveform optical output signal orsignals.

More specifically, optical repeater 93 includes input device 401comprising a photo detector connected in series circuit with the baseelectrode of low noise transistor 403 (a type 2 N 3117). Output fibers105 from first mixer 89 (shown in FIG. 1) provide the input to photodetector 401. All three of fibers 105 carry the same signal and threehowever are used since the combined power will result in a highersignal-to-noise ratio at receiver 91 output which is provided atterminal 407 of a.c. coupling capacitor 409. Photodetector 401 compriseseither a PIN photodiode or an avalanche photodiode. Photodetector 401 isreversed biased by photodetector power supply 413 which is connected tofirst electrode 473 of photodetector 401. Light entering photodetector401 results in a small photocurrent which is first amplified by cascodeamplifier circuit 416 which includes low noise first transistor 403 andsecond and third transistors 419 and 421, and further amplified bytransistors 423, 424, 425, and 427 of receiver circuit 91. Cascodeamplifier circuit 416 includes an output terminal 451 which is connectedthrough feedback network 481 (including parallel connected resistor 478and capacitor 499) to second electrode 457 of photodetector 401.Receiver circuit 91 is a.c. coupled through capacitor 377 connected inseries circuit with output terminal 451 of cascode amplifier circuit 416and capacitor 409 connected in series circuit path with output terminal407 of receiver circuit 91, which a.c. coupling prevents drift problemsassociated with temperature variations which normally would occur ind.c. coupled amplifiers. Series connected capacitor 409 is also utilizedto provide base line adjustment of the received signal by injection ofdroop in the waveform. Variable resistor 501 connected to base electrode503 of second emitter follower stage 507 of discriminator circuit 511provides means for varying the detection threshold voltage. Firstemitter follower stage 519 and aforementioned second emitter followerstage 507 introduce the same voltage offset to both the signal and thethreshold thereby providing a low impedance to comparator 523 inputs 524and 525 in discriminator circuit 511. When the signal voltage exceedsthe threshold voltage, comprator 523 provides a low level output andtransmitter 528 driver circuits 529 switch to an "on" conditionrespective light emitting diodes 531 thereby providing correspondingoutput optical signals 115 as inputs to second mixers of other couplers(not shown). Electrical test input terminal 537 provides a usefulfeature in permitting adjustment via variable resistor 501 (hereinbeforediscussed) and setting of the optical repeater detection threshold.Preferably, the threshold should be set high enough that signals 111from other couplers (as seen in FIG. 1) via input fibers to second mixer90 and multiple reflections within the coupler are rejected, and lowenough that the lowest level signals anticipated from local transmitters(signals 101 in FIG. 1) are repeated without error.

Turning now to FIG. 4, a 48 terminal data bus comprising four activemultiport fiber optic data bus couplers 88 is seen wherein pairedtransmitter output 115 and signals from other couplers 111 are seencoupled between couplers 88. A more detailed schematic version showingdetailed internal coupler 88 connections is shown in FIG. 5 while FIG. 1and the aforementioned detailed description in connection therewith maybe referred to for a complete understanding of the operation of opticalrepeater 93, first, and second mixers 89 and 90.

In the interest of consistency between FIGS. 4 and 5, it should beobserved that in FIGS. 4, 12 paired input and output ports (eachrepresented by a line with paired arrowheads) for each coupler 88 areshown, while in FIG. 5 the number of connectors 605 is only six sincefour lines or two pairs are brought out to each of connectors 605coupled by optical fibers (not shown) to and from local receivers andtransmitters.

A further figure, FIG. 6, has been included and leading to a furtherunderstanding of the present invention since giving in detail legendsdenoting all incoming and outgoing signals and signal paths intermediatea pair of couplers 88 of a data bus.

FIGS. 7, 8, 9, 10 and 11 have been included since exemplary of a methodof making active couplers 88 having certain unique features andadvantages hereinafter described. FIG. 7 shows a pair of identicallyshaped fixtures comprising top fixture 779 and bottom fixture 777 whichare utilized to position, retain, and align the several elements ofcoupler 89 (seen in FIG. 10 as including a pair of fiber assemblies 705and first and second mixer fiber assemblies 707 and 707). Top fixture779 and bottom fixture 777 are generally L shaped with the bottom foot(short leg 778 of the L having a step height h less than the thickness m(seen in FIG. 8) of the fiber or mixer array to be confined so that thetightest fit (no extra space between the fibers) is achieved. The innersurface 781 of foot 778 is cut in at 30 degrees so that the innersurface or top 781 of foot 778 forms an angle of 120 degrees withrespect to the back surface 783 of bottom fixture 777. Bottom fixture777 and top fixture 779 comprise a transparent material, e.g., precisionmolded plastic or glass drawn from a preform so that subsequent tosandwich assembly of fixtures 777 and 779 about the fiber array (asshown in FIG. 8), viewing can be had during final assembly. Thedimension t (thickness of the long side of L shaped fixture 777) iscritical in terms of providing for vertical alignment of mixer and fiberassemblies with respect to baseplate 931 (as seen in final couplerassembly in FIG. 10). Optical mixer fiber assembly 89 or 90 shown inFIG. 9 comprises a glass core portion 989 surrounded by a layer ofcladding material 981 (of lower index of refraction).

Fiber assembly 705 is formed as shown in FIG. 8 by first disposing thetwo-layer fiber array shown between top and bottom fixtures 779 and 777and then compressing the two layer fiber array by pressure againstfixtures 777 and 779 in an inward direction towards the plane of thefiber array with subsequent second step inward pressure then applied (asrepresented by arrows 333) in a direction perpendicular to inner surface781 of feet 778 and 779 with the subsequent curing of the two layerepoxy potted array for permanent retention thereof. Grinding andpolishing of the end faces of the two layer fiber array completes themethod of making fiber assemblies 705 later utilized in the finalcomposite coupler assembly of FIG. 10. Mixer 89, 90 assembly is shown inFIG. 9 similarly utilizing the aforementioned method and top and bottomL-shaped fixtures 779 and 777.

Final assembly of active coupler 88 is shown (top view) in FIG. 10 and(in perspective) in FIG. 11. As observed, two mixer fiber assemblies 89and 90 and two fiber assemblies 705 are aligned to form the finalassembly. Vertical alignment is achieved by assembling two of elements705 and elements 89 and 90 on common base plate 931. Lateral alignmentis achieved by visual observation of the aforementioned elements throughtransparent positioning fixtures. Elements of the final assembly formingactive coupler 88 are permanently secured in an integral structure by anoptical grade epoxy (not shown for clarity) to avoid loss and minimizeFresnel reflections at junctions between elements.

I claim:
 1. A method of assembling an array of optical fibers comprising:preparing an epoxy potted multilayered array of optical fibers; selecting a pair of generally L-shaped transparent fixtures with a bottom foot of said L-shaped transparent fixtures having a step height less than the thickness of said multilayered array and the inner surface of said bottom foot cut in at about 30 degrees; sandwiching said multilayered array between said pair of generally L-shaped transparent fixtures; compressing said multilayered array of optical fibers by applying pressure against said pair of generally L-shaped transparent fixtures in an inward direction towards the central plane of said multilayered array of optical fibers; applying further inward pressure in a direction perpendicular to the inner surface of the feet of said generally L-shaped transparent fixtures; then, curing said epoxy potted multilayered array of optical fibers to provide an integrated multilayered array of said optical fibers. 